The application is based upon and claims priorities from Japanese Application Nos. HEI 3-273023 filed Jul. 23, 1991, and HEI 3-205246 filed Aug. 15, 1991, the disclosure of which is incorporated by reference herein.
The present invention relates to a zoom lens system suitable for use with a small television camera such as a CCTV camera. More particularly, the present invention relates to a zoom lens system of this type that is capable of focus adjustment. The present invention also relates to a zoom lens system with a zoom ratio of 6 to 12 that is suitable for use with a small television camera such as a CCTV camera.
One of the requirements to be satisfied by small television cameras of the type contemplated by the present invention is to use a compact and high-resolution imaging device. To meet this requirement, a taking lens system with a small F number is needed.
If the F number of a lens system is reduced, its depth of focus decreases to such an extent that errors in mounting the imaging device will sometimes become greater than the depth of focus of that lens system. In this case, focus adjustment is necessary after mounting the imaging device.
It often occurs that the same lens system is used with different kinds of television camera and, depending on the specific kind of camera, filters such as a faceplate, an IR cutoff filter and a low-pass filter may or may not be provided in front of the image plane or their thickness will vary and, as a result, the focus of the lens system must be adjusted in accordance with the thickness of the filter used. In conventional television cameras, focus adjustment is performed for an individual camera by moving the lens system as a whole when it is mounted in the camera.
However, this conventional method of adjustment has the problem that when a lens system of interest is to be mounted in different kinds of television cameras aberrations occur due to the different thicknesses of filters, making it difficult for the lens system to exhibit its performance fully. An other requirement to be satisfied by small television cameras of the type contemplated by the present invention is to make the overall size compact enough to be compatible with the smallness of imaging devices they use. Further, in order to insure adequate space for electrical parts such as an electric motor and a diaphragm stop unit, the zoom lens system is desirably rendered as compact as possible.
However, heretofore known four-group zoom lens systems with an aperture ratio of 1:1.2 and zoom ratios of ca. 6 to 12 are disadvantageous in that the fourth lens group is composed of at least 7 lens elements or that its overall length is too great.
In addition, conventional zoom lens systems have a diaphragm stop provided between the third and the fourth lens groups or within the fourth lens group at a position comparatively close to the object and, hence, the diameter of a light beam passing through the diaphragm stop is large enough to require a bulky stop mechanism.